Address various linting issues

[ZetaTwo]

• Remove error return type from processingWorker as it is always nil and never checked.
• Use variable read from locked dict
• grpc.WithInsecure is deprecated: use WithTransportCredentials and insecure.NewCredentials()
• Check return value of io.Copy
• Rename importer template to stop treating it as Go code that's used
• Check return value of extractWimFolder()
• Use bytes.Equal(...) instead of bytes.Compare(...) == 0

[meeehow]

This looks good, just one small request, take a look at my last comment

[meeehow]

Thanks!